This invention relates to the production of sulfonated aromatic hydrocarbons. More particularly, this invention relates to an improvement in the process for producing a disulfonic acid of an aromatic hydrocarbon and a mono-sulfonic acid of an aromatic hydrocarbon.
Benzene disulfonic acid is prepared with good yields by reacting benzene or mono-sulfonated benzene with an excess amount of inhibited fuming sulfuric acid (oleum). The benzene disulfonic acid product contains only a small amount of water produced from the sulfonation reaction but it does contain unreacted, sulfuric acid along with by-products such as benzene mono-sulfonic acid, other sulfonated acids of benzene, sulfones, and other by-products. The separation of the unreacted sulfuric acid from the disulfonated benzene at the completion of the reaction is difficult and expensive.
One method of separating the unreacted sulfuric acid from the benzene disulfonic acid is described in Canadian Pat. No. 784,253 by John O'Brochta entitled, "Process for Separation of Sulfonated Benzenes". This patent discloses separating the unreacted sulfuric acid from benzene disulfonic acid in a benzene disulfonic acid product by passing the product as a film of 0.5 to 3.0 millimeters through an evaporation zone at a pressure of less than 0.10 mm of mercury and at a temperature of 200.degree.-240.degree. C. In an alternative embodiment, the product is reacted with benzene to convert any unreacted sulfuric acid to benzene mono-sulfonic acid. The water formed during this reaction is continuously removed by formation of a water-benzene azeotrope which is distilled from the reaction. The benzene mono-sulfonic acid is separated from the benzene disulfonic acid in the evaporation zone.
In the above-described process the water formed from the reaction of the unreacted sulfuric acid in the benzene disulfonic acid product was continuously removed to enable the reaction to go to completion. This is done on the theory that if the water is not eliminated from the reaction, the water formed by the reaction would dilute the concentration of sulfuric acid. This dilution would cause the rate of reaction to decrease until a limiting concentration of sulfuric acid is reached and the reaction stops. This dilution effect is common in a sulfonation reaction of an aromatic compound with sulfuric acid.
Several methods have been developed to overcome the dilution effect of the water formed during the course of a sulfonation reaction where sulfuric acid is used as the sulfonating agent. These methods include physically removing the water from the reaction or chemically combining the water to lessen its effect on the reaction. In the above-described process the water of reaction was removed by the formation of a benzene-water azeotrope which involves the use of more benzene than that needed to react with the unreacted sulfuric acid. This operation is time-consuming and increases the cost of the process. Also, this operation leads to an increase in the production of sulfone by-products.
The primary objective of the improvement of the present invention is to provide a process for reacting any unreacted sulfuric acid sulfonating agent present in a disulfonic acid product produced by reaction of an aromatic hydrocarbon and a strong sulfonating agent with the aromatic hydrocarbon to produce the mono-sulfonic acid of the aromatic hydrocarbon without the need for removing the water formed during the course of the reaction.